12V, 5V Solid-State Power Supply Using IC LR645

Here we study how the mains voltage is controlled to 12V and 5V using just a single IC LR645G and some other supportive ordinary active semiconductors.


In one of my earlier posts I provided a similar circuit but it utilized a high voltage capacitor for dropping the mains voltage to lower usable levels.

Thanks to Supertex ic. for providing us with this wonderful little chip LR645G, which single handedly controls any voltage between 24 and 270 V AC and produces DC voltages below 15 volts at the output, which becomes ideally suitable for operating sensitive, compact electronic circuits.

The best part of the circuit is that it does not incorporate any bulky of heavy components like a transformer or non polar high voltage capacitors.

Though we all know the simple way of constructing transformerless power supply units using high voltage capacitors, these high voltage capacitors have one big drawback.

At switch ON, these caps allow high surge inputs to pass through them and also intermediate transients become unstoppable with these devices.

The drawback can cause havoc with any electronic circuit that may be connected to  such power supply configurations.

Using LR645G

Using LR645G the above threat becomes absolutely nullified.  The maximum current available from this device is quite low, around 3 mA, however that's never a problem, because the current can be shot up to 150 mA, through a simple addition of an  fet DN2540N5 in the circuit.

The figure shown above is a classic circuit set up of a 12V and 5V transformer less power supply circuit which can provide outputs of 15 volts and 5 volts.

15 volts is available just at the junction of the output of LR645 and the input of Ic 7805.
If the 5 volt option is not required, the configuration around the 5 volt regulator can be just eliminated, which makes the circuit yet simpler and compact.

How the Circuits Works

In short the circuit diagram may be understood in the following manner:

  • The high voltage AC mains is rectified by the bridge configuration using four diodes at the input.

  • The rectified voltage is smoothed by the filter capacitor introduced just after the bridge network.

  • The rectified, filtered high voltage is fed to the IC LR645LG, which effectively reduces the voltage to 15 volts at 3 mA.

  • The FET pulls the 3 mA current output to 150 mA and feds it to the next stage which incorporates the 5 volt regulator stage.

However one big drawback of eliminating the use of a transformer is the DANGER of high voltage shock that actively hangs with all the naked points of the circuit.

Therefore extreme caution must be exercised while building and testing this circuit and other attached circuits.

Parts List

Diodes - 1N4007

Input Capacitor - 4.7uF/400V,

Output Capacitors are 1uF/25V

ICs are LR645LG and 7805,

FET - DN2540N5

Need Help? Please send your queries through Comments for quick replies! And please Bookmark my site :)


Swagatam said…
Thanks! Actually the conditions are specified by the chip manufacturer so I am not sure if any other device could be replaced with the given FET, definitely not a transistor, at the most we could try an equivalent high current n-FET.
kathiravan said…
swagatam sir can we use an isolation transformer at the ac phase input side of circuit to eliminate its drawback.
Swagatam said…
Hi Kathiravan,

If you use a transformer then you won't need the above circuit, simply put a bridge and a 78XX regulator with your trafo output :- your power supply is ready.
Avik Paul said…
Avik Paul said…
One more question sir, in this circuit i connect in put volt with middle pin of that FET - DN2540N5 and also connect gate pin of that LR645LG...

PLZ clear my confusion......
Swagatam said…
i did not understand your question, the FET has a drain, gate and a source....identify these terminals correctly and then connect them exactly as given in the schematic.
Avik Paul said…
Yes I know that FET - DN2540N5 Pin configuration..
1st pin - Gate
2nd Pin - Drain
3rd pin - Source
My question is in this circuit which pin i connect where....
Swagatam said…
drain is connected to bridge...source to Vout
Bharani Daran said…
Dear Sir,

we need 24v /300mA transformerless power supply circuits diagram
Swagatam said…
Dear Bharani,

you can try the following design:

Bharani Daran said…
Dear sir,

can i use Z1 24v 1 watt zenner doide?
Bharani Daran said…
Dear Sir,

If i can use 7824 which component replaced or modify?
Swagatam said…
7824 cannot be used in the above circuit.
Bharani Daran said…
Dear Sir,

I have 7w SMD LED PCB avilable.(Spec is 24v/300mA) if i try use above circuit diagram or not ?
Swagatam said…
Dear Bharani, the maximum available voltage from the above circuit is 15V, therefore a 24V led will not work, and anyway 7watt is way too high it will require an smps circuit.
Bharani Daran said…
Dear Sir,

ok sir, anyway i need 24v/300ma transformerless power supply or SMPS circuit.


Bharani Daran said…
Dear Sir,

Any SMPS circuit avilable in 24v/300mA
Swagatam said…
Dear Bharani,

Please refer to the following links, you can easily tweak the output shunt zener resistor for getting 24V:



GenWockZ said…
Good day sir Swagatam .. Can I suggest that can you make a schematic diagram of a variable power supply 0-6v with a Surge current protecter ?

Thanks in advance :)
Swagatam said…
thanks Ge, I'll try to upadte one soon for you...
Swagatam said…
...thanks GenWoc
GenWockZ said…
Thank you so much sir .
rahul kumar said…
Use Buck Converter
It need astable multivibrator
small inductor
and capacitor

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